The invention relates to a method and arrangement for determining the location of partial discharge sources in an electric system.
The voltage strength of an insulating structure refers to its ability to endure voltage stress without electric discharges that cause disturbances or damage. If the voltage stress in an the insulating structure is increased sufficiently, discharges occur which make the insulation completely or partially conductive. The latter are called partial discharges. A partial discharge does not unite the electrodes and, thus, the insulating properties of the insulating material do not completely disappear. Partial discharges do, however, wear the insulating material and thus further weaken: its voltage strength and may finally lead to a complete electric discharge. Partial discharges can be divided into two main groups, internal and external discharges. Internal discharges comprise cavity discharges and external discharges comprise surface, corona and spark discharges. Each group can further be divided into several subgroups which are often difficult to clearly distinguish from each other. Partial discharge pulses are very fast pulses and usually occur as pulse groups. A partial discharge and the reversal of charge that occurs in connection with it show as a current pulse in the connectors of the insulating material. In practice, these current pulses also Sum into the phase voltage of the system.
Locating partial discharge sources quickly in an electric system is important for preventing the occurrence of a more serious defect. Detecting partial discharges in electric networks and locating the discharge source has usually required an analysis of measuring data by a person skilled in the art. There are also methods for determining the location of the discharge source automatically.
Published application WO 93/17351, for instance, discloses a method of locating partial discharges in a cable, based on the use of a measuring sensor and device. The measuring device measures the partial discharge pulse coming directly to the sensor from the fault location and the partial discharge pulse from the fault location reflected to the sensor from the other end of the cable. The location of the discharge in the cable is determined on the basis of the time difference of the pulses. The method is called a TDR (time domain reflectometry) method.
Published application CA 2119702 discloses a method of determining the location of partial discharges in a cable without methods based on pulse reflections (TDR methods) or communication between sensors. The method is based on using two sensors installed close to the ends of the cable, for instance. The first sensor measures on one side of the fault location the partial discharge pulse coming directly from the fault location, which pulse is called the first pulse. The second sensor measures on the other side of the fault location the partial discharge pulse coming directly from the fault location, which pulse is called the second pulse. Immediately after detecting the second pulse, a third pulse which can be measured with the first sensor is sent to the cable. By calculating the time difference between said first and third pulse, it is possible to determine the location of the discharge point.
A drawback with the prior art disclosed in published application WO 93/17351 is that it may function poorly if the cable section is long or has high attenuation or if there is a lot of interference in the environment. This is caused by the fact that in the worst case, the reflected pulse needs to propagate a distance of almost twice the length of the cable and attenuates too much to be measurable. In other words, if the locating is to work in all situations, the length of a cable section covered by one measuring point remains at approximately half of what would be possible with another method not based on measuring pulse reflection.
A drawback with the prior art disclosed in published application CA 2119702 is that especially in conditions having interference, the amplitude of the reference pulse (third pulse) fed through the sensor to the cable must be high enough, which may cause problems in implementing the sensor and the pulse generator, and consequently, their price becomes easily unreasonably high. If the sensor is designed adequate with respect to measuring sensitivity and advantageous in price, its voltage division ratio will not be suitable to feeding a sufficiently strong reference pulse to the cable.
It is thus an object of the invention to develop a method and a system implementing the method so as to solve the above-mentioned problems. The object of the invention is achieved by a method and system characterized in what is stated in the independent claims 1 and 8. Preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the fact that the properties of partial discharge pulses generated in a partial discharge source change as a function of distance when they propagate in the electric system along a conductor, for instance. By deriving characteristic parameters depicting certain properties of partial discharge pulses from measured partial discharge pulses and by placing them in a model formed in advance, for instance experimentally, which describes the change of the characteristic parameters as a function of distance substantially at the frequencies where partial discharge pulses occur, it is possible to determine the distance of a partial discharge source from the measuring point at a certain accuracy.
The method and arrangement of the invention provide the advantage that by means of the invention, it is possible to automatically detect and locate with a certainty and accuracy partial discharge sources occurring in an electric system. In addition, the invention can easily be applied to monitoring different apparatuses and environments by changing the used model to correspond to the system to be monitored. Further, the method and system of the invention does not require additional equipment in addition to the measurement of partial discharge pulses made at one point.